I'd also recommend switching off EVERYTHING that is not needed.
 
Run as bare bones a system as you can - pc, monitors, interface &, er, monitors. Nothing else. Try to ensure that all mains cables are connected as close as possible to a Star System so that everything only comes out of one wall outlet.
 
If this DOES alleviate the problem you can identify the culprit by plugging stuff back in, one at a time until the noise comes back

Update. The Focusrite 6i6 is used with the power plugged up.

Slightly off-topic, but...
I recently discovered that any project containing the softsynth GSI VB3 (Hammond Organ) emits a constant pulsating noise even when idle. It is only noticable when the volume is turned up high and disappears when the synth is turned off. Also I believe that the UAD EMT250 reverb is hissy when idle, although I've never checked this out. Maybe there are other noisy plugins too.

I was having an issue where I can actually hear the hard drives chugging as well as other noise. It seemed to be a grounp loop issue. What I did was used a  Prong to 2 Prong Grounding Converter. One one my interface and one on my speakers and all went quiet. Don't know if this could be your issue but it may be worth a try. Here is a link about ground loops which is what led me to investigate groung loop issues.
https://www.youtube.com/watch?v=4YM1iwC6vhg
 

Bristol_Jonesey
I'd also recommend switching off EVERYTHING that is not needed.
 

that includes air conditioning, heating, cell phones, burglar alarms, missile defence systems, news on politics, news on economy, vegan cooking, divorce lawyer communication, ... 

He's got a missile defence system in his studio ????
 
Wow, way to go.......

Bristol_Jonesey
He's got a missile defence system in his studio ????
 
Wow, way to go.......

if you haven't got one yet, I'm sure you can get one online real quick ;-) sweetwater, thomann, amazon, ... just don't fall for the next monthly subscription fee

Bristol_Jonesey
He's got a missile defence system in his studio ????
 
Wow, way to go.......

No those were not missile defense rockets.  Those were two lava lamps

This is a grounding problem in the PC or one of the components. This can be hellishly difficult or impossible to fully solve since it can be a combination of issues causing the problem.
My old DAW had this issue where I would get RF noise leaking into my signal through the audio interface. In my case it definitely had to do with some Roland gear I was plugging into my Audio interface. 
 
I ended up reducing the noise a lot by switching off all dimmer circuits as well as using ferrite cores on all cables in the system. I also use a HumX which works fairly well. However even with that I would randomly sometimes pick up weird RF noise. It turned out that there was a radio station within 5 miles of my house. I had an electrician come over once and he told me someone complained of voices in their house - it turned out the signal was so strong that wires in the house were actually inducing and acoustically playing back radio. I didn't even know that was possible :-P

That's quite interesting Noel. Do you know if this is mainly a PC issue or does it happen with Macs equally?